


Expanding the Relationship

by JellyFicsnFucks



Category: Undertale (Video Game)
Genre: Alternate Universe - Underfell (Undertale), Be warned for language, Crying, Established Relationship, Feelings, Hurt/Comfort, Implied Relationship: SpicyHoneyMustard, Implied Sexual Content, M/M, Mention of sexual acts, Mild Language, No Sex, Sibling Incest, implied threesome
Language: English
Status: Completed
Published: 2020-10-27
Updated: 2020-10-27
Packaged: 2021-03-08 18:48:30
Rating: Mature
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 1,569
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/27231493
Author URL: https://archiveofourown.org/users/JellyFicsnFucks/pseuds/JellyFicsnFucks
Summary: “I want to date Swapshit” was the first words Boss had said that morning. Red and him stood in front of the bathroom mirror brushing their teeth when the news finally hit hard. His brother" ... was replacing him.
Relationships: Papyrus/Sans (Undertale), Relationship: Red and Edge, Relationships: Fellcest
Comments: 3
Kudos: 36





	1. Chapter 1

“I want to date Swapshit” was the first words Boss had said that morning. Red and him stood in front of the bathroom mirror brushing their teeth when the news finally hit hard. His brother stuttered in place, the edges of his mouth turning south- then north- then south again before finally settling on a scowl. They stared back at their reflections, not bothering to turn to the other while they spoke. 

“Yeah?” Red asked. He smothered his toothbrush in more paste and became busy with checking his own reflection, watching himself with caution. He smiled again, naturally letting it fall on his face while he brushed his sharpened fangs. He made sure to relax his shoulders, curve his posture, slouch with the news like it meant nothing. Hiding behind that casual smile he always wore when Boss had news. “Cool.”

“Do you approve?” Boss asked. He bent to wash his own face, splashing water on his side of the sink. There was a long silence when Red said nothing. Could say nothing. His insides churned with words that failed to leave his mouth, and with the toothbrush in his mouth he had the perfect alibi for the silence. So when Boss rose back up and toweled off to see his dearest brother in the mirror there was an explained reason for the prolonged answer that Red struggled to give. 

He pretended nothing was wrong. He stared at his own reflection, his nervous beating heart and quaking hands weren’t obvious in his demeanor. He globbed more toothpaste onto his brush and tried to buy more time. But Boss could read him like an open book. 

“That bad - huh?” Fell heaved a sigh and reached for his own toothbrush. “Swapshit likes you, you know. I was hoping you’d get along with him.” 

Red couldn’t prolong his questions any longer. He spit up into the sink, not caring about the mess of white froth on his chin. “What about us?!” He turned to Fell at once, needing to see his face as if the answer was already written on his forehead. “Are you breaking up with me??”

Fell doesn’t return the same courtesy of turning to him. He stares borishly at the mirror reflection and raises a bone brow in question. “Dear brother, do I hear jealousy?” He smirked, which earned him a hard punch in the ribs.

“I’m your boyfriend!!” Red shrieked. “Why wouldn’t I be jealous?!!!”

“Boyfriend? We’ve never gone on a date, brother.”

“-That doesn’t matter when I’m eating your dick everyday!!” 

“Language..” 

“FUck NO! You don’t get to pretend like our relationship isn’t important Boss! How could you even think I would ever be okay with it!? Am I just a fucking toy to you!? A pet?!” He looked to his reflection, wiping the spittle from his mouth and straightening his posture to be slightly taller. “I’m not your fuck toy!” Red snaps his hands to the back of his neck and unclasps the red collar that's been put on him. He slams it on the bathroom sink and storms out the room. 

Leaving Boss alone. 

At last the skeleton breaks his view of the mirror and looks down at the sink. His bare fingers hover over the collar, afraid of touching it. He looks over his shoulder at the slammed door and turns to it. Opening it slowly in case Red was leaning on the door like he’d done on other fights, but the skeleton was gone. It didn’t take long for Fell to figure out where he’d gone. The front door was still locked with several dozen padlocks and Red didn’t bring his shoes so he couldn’t have teleported away. Must be stewing in their bedroom. 

The skeleton turned around and finished his morning routine in front of the mirror. Returning only when he’d figured Red had enough time to stop being angry. 

He opened the door slowly, not bothering to knock like they had done with they were baby bones. That boundary was quickly crossed when they shared the same bed each night. Fell strided over to the bedside and pulled down the clump of blankets from the crying monster trying to hide himself inside a pillow fort. 

“Sans.” Fell sighed... he drew his claws to pet at his brothers scratched skull. The red color of tears stained the pillow below him. “You should know better than anyone I’d never replace you. Not when the royal guard told me to abandon my weakest link… not when Asgore decreed monsters need a health over 5 to be free. And certainly not for Swapshit. That's why I asked you first, dear brother. “ He leaned down and pecked a skeleton kiss along the others vertebrae, trying to make Red show his face. 

“I love everything about you Red… your body… your soul… their mine.” 

“Yousaidimnotyourboyfriennn~” 

“Of course you’re not dumbass.” Fell chuckled, he picked up Red hand and kissed each knuckle. “You’re my dearest brother… our bond is so much deeper than that. I just wanted to spice things up a bit... you can’t blame me.”

“... with Stretch?”

“Swapshit has been aggravating me lately, it's so exciting to want to punch him and yet not being able to… he challenges me with his words… and its refreshing to have a new challenger. I want to break him of course.” 

“...”

“And having you spread out...mmn... taking us both like a champ…. That would be beautiful. I can’t wait to see your crying face… begging for more.” 

“You don't mean that.”

“Really...” Fell leaned forward, scooping Red up in his arms. So receptive, Red melted into the touch, and clinged onto Boss’s shirt. Mouth twitching for another kiss, eyes pink from crying… adorable. 

They kissed deeply again, Fell whispering how they could tie him to the bed while he fucked the brains out of swapshit. 

Whispering how Red could sit on swapshits face or ride him as he watched. Red flushed with embarrassment, never once saying no to an idea. 

He openly wondered what his moans would sound like. Would they be mewls of pleasure or deep guttural groans? 

Would he taste like Honey? Or would he taste like cigarettes? Would his kisses be sweet? Would he bite?

Would a date go slow? Fast? Would all that frustration from fighting lead to something more intense and desperate?

What was the standard amount of time to date before asking for a threesome?

“He’ll be such a great partner for us… don’t you think?” 

Red nodded slightly. Not wanting to shut down the idea at a threesome completely. But- “You’ll date Stretch?” He asked softly. “Would you love him? Love him more than me?”

Boss smiled widely, his eyes smirking with the rise of his cheek bones. He pulled the collar back out and clasped it gently around his brothers neck. The rounded spikes and red leather matched the glow of his magic, the hue of his blushes. He leaned forward again, not to kiss Red but to hold him firmly against his own bones. “Sans…” He whispered, breath hot on his spine. “I’ll always love myself more than you. I mean, I am great afterall.”

“You jerk!” Red laughed. He pounded his fist against his ribs again, but there wasn’t as much power in it as the hit earlier. He laid his head on Boss’s shoulder and hugged him back. “I love you.”

“... I love you too.” Fell whispered. “Now…” He straightened up and looked his brother in the eye. “ … since your feeling left out, let’s go on a date. You and me.”

“But... Stretch?”

“He can wait…” Fell smirked, pulling at Red’s chin to see the small pink tongue that easily drooped forward. Wanting another kiss, he couldn’t deny the tiny skeletons wishes. He claimed that mouth as he had so many other times. But this one felt new, special. A trail of saliva broke off from their heated exchange. “I’ll always love you more.”


	2. bonus

\--------------

“...” Stretch couldn’t place his finger on it… but he knew something evil was being plotted by the Fell brothers. Somehow they seemed... happy. And that wasn’t normal for the edgy fuckers. Stretch crossed his arms and stared at them from the kitchen countertops. Blue, Papyrus, Red, and Fell were all playing a word association game or something, but he could care less. Something was… unsettling about those smiles. 

“Bothered too?” Classic poured him a cup of coffee and took the chair next to him, overlooking the fun times being had in the next room. The little kitchen window was a nice break to both be present in the room… and distanced from the game. “Red’s been bothering me nonstop with shipping names. It's been… disturbing.” 

“Shipping?”

“Like how he calls me Ketchup sometimes… and sometimes we call him mustard? … He combined our names for some fantasy relationship in his head. Real sick.” 

“Hmm.” Stretch stirs his coffee, idly thinking about it. “I don’t think Red likes you like that though. I think… he and Fell are a couple. Aren’t they?” 

“Couple of freaks. Yeah.” Sans sips his coffee. Laughter makes it's way across them and a million monopoly dollars are flung across the living room. They aren’t even playing monopoly… Sans sighs. “At least they’re having fun.” 

“I think Fell said something similar before.” Stretch paused. “Spicyhoneymustard. What the hells that supposed to mean?” 

“...means you're fucked. Dont it?” Sans sipped his coffee.


End file.
